Semiconductor wafers typically have a complex structure composed of a substrate on which a plurality of transistors has been formed. Integrated circuits are chemically and physically connected into a substrate by patterning regions in the substrate and layers on the substrate with various materials including metals, insulators, and semiconductors. In order to produce an operable semiconductor wafer and to maximize the yield, performance, and reliability of the wafer, it is desirable to polish selected surfaces of the wafer (e.g., a metal-containing surface) without adversely affecting underlying structures or topography. This complexity leads to difficulties in designing effective polishing systems for semiconductor wafers. In fact, various problems in semiconductor fabrication can occur if the process steps are not performed on wafer surfaces that are adequately flat and uniform (planarized). Because the performance of a semiconductor wafer is directly associated with the planarity of its surface, it is crucial to use a polishing composition and method that results in a high polishing efficiency, uniformity, provides a high material removal rate, and leaves a high quality finish with minimal surface defects. In many cases, the hardness and chemical stability of the various materials making up the wafer can vary widely, further complicating the polishing process.
Conventional polishing systems and polishing methods typically are not entirely satisfactory at planarizing metal-containing semiconductor wafers. In particular, polishing compositions and polishing pads can have less than desirable polishing rates, and their use in the chemical-mechanical polishing (CMP) of semiconductor surfaces can result in poor surface quality. This is particularly true for noble metals such as ruthenium, which is utilized in fabricating high performance semiconductor devices and capacitors in dynamic random access memory (DRAM) devices.
Current ruthenium polishing compositions typically rely on relatively hard abrasives such as α-alumina, and strong oxidizing agents, such as oxone or ceric ammonium nitrate, to provide adequate ruthenium removal rates (e.g., at least about 100 Angstroms-per-minute (A/min) Ru removal rate). This is due, at least in part, to the high degree of chemical inertness and strong response to mechanical abrasion exhibited by ruthenium barrier layers. Typically, relatively weak oxidants such as hydrogen peroxide are not very efficient in ruthenium polishing processes, requiring long polishing times and a high polishing pressure in order to adequately planarize the ruthenium.
Conventional ruthenium CMP compositions that utilize strong oxidants, such as oxone or ceric ammonium nitrate, work by oxidizing the ruthenium to relatively high oxidation state ruthenium species, such as toxic and undesirable ruthenium(VIII) tetroxide (RuO4), and concomitantly dissolving the oxidized ruthenium species from the surface of the substrate, aided by the hard abrasive (e.g., α-alumina). Use of milder oxidants (e.g., hydrogen peroxide), and the use of softer abrasives (e.g., silica or titania), generally are not practical, due to the relatively low ruthenium removal rates (e.g., <100 Å/min) achieved under such conditions. Accordingly, uniform CMP of patterned wafers can be difficult to achieve, due to the harsh conditions typically required for ruthenium removal, which can result in over-removal of other materials, such as silicon dioxide (e.g., plasma enhanced tetraethylorthosilicate-derived silicon dioxide, PETEOS), resulting in poor surface quality.
Many compositions and methods for chemical-mechanical polishing (CMP) the surface of a substrate are known in the art. Polishing compositions (also known as polishing slurries, CMP slurries, and CMP compositions) typically contain an abrasive material in an aqueous carrier. A surface of a substrate is abraded to polish the surface by urging the surface of the substrate into contact with the polishing pad at a selected force (down force) and moving the polishing pad relative to the surface while maintaining a CMP slurry and oxidizing agent between the pad and the surface of the substrate. Typical abrasive materials include silicon dioxide (silica), cerium oxide (ceria), aluminum oxide (alumina), zirconium oxide (zirconia), titanium dioxide (titania), and tin oxide. U.S. Pat. No. 5,527,423 to Neville et al., for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,489,233 to Cook et al. discloses the use of polishing pads having a surface texture or pattern, and U.S. Pat. No. 5,958,794 to Bruxvoort et al. discloses a fixed abrasive polishing pad.
In conventional CMP techniques, a substrate carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The substrate is mounted on the polishing head. The carrier assembly provides a controllable pressure (down force) to urge the substrate against the polishing pad. The pad is moved relative to the substrate by an external driving force. The relative movement of the substrate and pad serves to abrade the surface of the substrate to remove a portion of the material from the substrate surface, thereby polishing the substrate. The polishing of the substrate by the relative movement of the pad and the substrate typically is further aided by the chemical activity of the polishing composition and/or the mechanical activity of an abrasive suspended in the polishing composition. It is highly desirable to utilize polishing compositions that are compatible with CMP apparatus as described herein.
The harsh conditions typically required for ruthenium removal can result in undesirable separation of the ruthenium layer from the interlayer insulating layer, as well as dishing and erosion effects on the ruthenium layer adjacent to the interlayer. In many cases, it is also desirable for Ru CMP compositions to provide relatively high silicon dioxide removal rates (e.g., at least about 100 A/min, preferably about 200 A/min or greater for removal of PETEOS), in addition to relatively high Ru removal rates and low defectivity.
There is an ongoing need to develop CMP compositions that are capable of polishing a semiconductor substrate, particularly a ruthenium-containing substrate, without the use of strong, RuO4-generating oxidants, and which exhibit relatively high ruthenium removal rates. The present invention provides such CMP compositions. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.